Antibodies are natural proteins that the vertebrate immune system forms in response to foreign substances (antigens), primarily for defense against infection. For over a century, antibodies have been induced in animals under artificial conditions and harvested for use in therapy or diagnosis of disease conditions, or for biological research. Each individual antibody producing cell produces a single type of antibody with a chemically defined composition, however, antibodies obtained directly from animal serum in response to antigen inoculation actually comprise an ensemble of non-identical molecules (i.e, polyclonal antibodies) made from an ensemble of individual antibody producing cells.
Hybridoma technology provided a method to propagate a single antibody-producing cell for an indefinite number of generations with a screening method to identify clones of cells producing an antibody that would react with a particular antigen. Development of this technology allowed production in unlimited quantities of structurally identical antibodies with essentially any desired antigenic specificity. Such antibodies are commonly called monoclonal antibodies, and most originate from rodents. Sequencing of monoclonal antibody genes allowed the primary amino acid structure of the antibody be defined.
The advent of recombinant DNA methodology enabled structural engineering of antibody genes and production of modified antibody molecules with properties not obtainable by hybridoma technology. In the therapeutic arena, one aim of this methodology has been to reduce the immunogenicity in humans of rodent monoclonal antibodies by modifying their primary amino acid structure. Reduction of the immunogenicity of therapeutic antibodies is desirable because induction of an immune response can cause a spectrum of adverse effects in a patient, ranging from accelerated elimination of the therapeutic antibody, with consequent loss of efficacy, to fatal anaphylaxis at the most extreme.
One strategy to reduce immunogenicity of foreign monoclonal antibodies has been to replace the light and heavy chain constant domains of the monoclonal antibody with analogous domains of human origin leaving the variable region domains of the foreign antibody intact. The variable region domains of the light and heavy chains are responsible for the interaction between the antibody and the antigen. The joining domains connecting variable domains to constant domains are situated in a region remote from the site of antigen binding, therefore, the joining domains between variable and constant domains generally do not interfere with antigen binding. Chimeric antibody molecules having mouse variable domains joined to human constant domains usually bind antigen with the same affinity constant as the mouse antibody from which the chimeric was derived. Such chimeric antibodies are less immunogenic in humans than their fully murine counterparts. Nevertheless, antibodies that preserve entire murine variable domains tend to provoke immune responses in a substantial fraction of patients. For example, INFLIXIMAB™, a widely prescribed chimeric antibody that is considered safe, induced a human anti-chimeric antibody response in 7 out of 47 Crohns Disease patients. (Rutgeerts, P., et al (1999) Efficacy and safety of retreatment with anti-tumor necrosis factor antibody (INFLIXIMAB) to maintain remission in Crohn's disease. Gastroenterology 117, 761-769).
That humans would mount an immune response to whole murine variable domains was predictable, thus, efforts to obtain variable domains with more human character had begun even before clinical trials of such standard chimeric antibodies had been reported. One category of methods frequently referred to as “humanizing,” aims to convert the variable domains of murine monoclonal antibodies to a more human form by recombinantly constructing an antibody variable domain having both mouse and human character. Humanizing strategies are based on several consensual understandings of antibody structure data. First, variable domains contain contiguous tracts of peptide sequence that are conserved within a species, but which differ between evolutionarily remote species, such as mice and humans. Second, other contiguous tracts are not conserved within a species, but even differ even between antibody producing cells within the same individual. Third, contacts between antibody and antigen occur principally through the non-conserved regions of the variable domain. Fourth, the molecular architecture of antibody variable domains is sufficiently similar across species that correspondent amino acid residue positions between species may be identified based on position alone, without experimental data.
Humanized strategies share the premise that replacement of amino acid residues that are characteristic of murine sequences with residues found in the correspondent positions of human antibodies will reduce the immunogenicity in humans of the resulting antibody. However, replacement of sequences between species usually results in reduction of antibody binding to its antigen. The art of humanization therefore lies in balancing replacement of the original murine sequence to reduce immunogenicity with the need for the humanized molecule to retain sufficient antigen binding to be therapeutically useful. This balance has been struck using two approaches.
In one approach, exemplified by U.S. Pat. No. 5,869,619 to Studnicka and by Padlan (1991) A possible procedure for reducing the immunogenicity of antibody variable domains while preserving their ligand binding properties, Molecular Immunology 28:489-498, characteristically human residues are substituted for murine variable domain residues that are determined or predicted (i) to play no significant chemical role in the interaction with antigens and (ii) to be positioned with side chains projecting into the solvent. Thus, exterior residues remote from the antigen binding site are humanized, while interior residues, antigen binding residues, and residues forming the interface between variable domains remain murine. One disadvantage of this approach is that rather extensive experimental data is required to determine whether a residue plays no significant chemical role in antigen binding or will be positioned in the solvent in a particular three dimensional antibody structure.
In another more general approach, exemplified by U.S. Pat. No. 5,225,539 to Winter and by Jones et al (1986) Replacing the complementarity determining regions in a human antibody with those from a mouse, Nature 321:522-525, contiguous tracts of murine variable domain peptide sequence considered conserved are replaced with the correspondent tracts from a human antibody. In this more general approach, all variable domain residues are humanized except for the non-conserved regions implicated in antigen binding. To determine appropriate contiguous tracks for replacement, Winter, and Jones et al (1986) utilized a classification of antibody variable domain sequences that had been developed previously by Wu and Kabat (1970).
Wu and Kabat pioneered the alignment of antibody peptide sequences, and their contributions in this regard were several-fold: First, through study of sequence similarities between variable domains, they identified correspondent residues that to a greater or lesser extent were homologous across all antibodies in all vertebrate species, inasmuch as they adopted similar three-dimensional structure, played similar functional roles, interacted similarly with neighboring residues, and existed in similar chemical environments. Second, they devised a peptide sequence numbering system in which homologous immunoglobulin residues were assigned the same position number. One skilled in the art can unambiguously assign what is now commonly called Kabat numbering, to any variable domain sequence, without reliance on any experimental data beyond the sequence itself. Third, for each Kabat-numbered sequence position, Kabat and Wu calculated variability, by which is meant the finding of few or many possible amino acids when variable domain sequences are aligned. They identified three contiguous regions of high variability embedded within four less variable contiguous regions. Other workers bad previously noted variability approximately in these regions (hypervariable regions) and posited that the highly variable regions represented amino acid residues used for antigen binding. Kabat and Wu formally demarcated residues constituting these variable tracts, and designated these “complementarity determining regions” (CDRs), referring to chemical complementarity between antibody and antigen. A role in three-dimensional folding of the variable domain, but not in antigen recognition, was ascribed to the remaining less-variable regions, which are now termed “framework regions”. Fourth, Kabat and Wu established a public database of antibody peptide and nucleic acid sequences, which continues to be maintained and is well known to those skilled in the art.
The humanization method disclosed by Winter and Jones using the Kabat classification results in a chimeric antibody comprising CDRs from one antibody and framework regions from another antibody that differs in species origin, specificity, subclass, or other characteristics. However, no particular sequences or properties were ascribed to the framework regions, indeed, Winter taught that any set of frameworks could be combined with any set of CDRs. Framework sequences have since been recognized as being important for conferring the three dimensional structure of an antibody variable region necessary to retain good antigen binding. Thus, the general humanizing methods described by Winter and Jones have the disadvantage of frequently leading to inactive antibodies because these references do not provide information needed to rationally select among the many possible human framework sequences, those most likely to support antigen binding required by a particular CDR region from a non-human antibody. Subsequent developments in the field have been refinements within the scope of Winter to deal with loss of avidity for antigen observed with some humanized antibodies relative to the avidity of the corresponding mouse antibodies. (Avidity is a quantitative measure of partitioning of an antibody, in the presence of antigen under conditions approximating chemical equilibrium, between free and antigen-bound forms. For reactions in solution not subject to multivalent binding effects, avidity is the same as affinity, the biochemical equilibrium constant.).
U.S. Pat. No. 5,693,761 to Queen et al, discloses one refinement on Winter for humanizing antibodies, and is based on the premise that ascribes avidity loss to problems in the structural motifs in the humanized framework which, because of steric or other chemical incompatibility, interfere with the folding of the CDRs into the binding-capable conformation found in the mouse antibody. To address this problem, Queen teaches using human framework sequences closely homologous in linear peptide sequence to framework sequences of the mouse antibody to be humanized. Accordingly, the methods of Queen focus on comparing framework sequences between species. Typically, all available human variable domain sequences are compared to a particular mouse sequence and the percentage identity between correspondent framework residues is calculated. The human variable domain with the highest percentage is selected to provide the framework sequences for the humanizing project. Queen also teaches that it is important to retain in the humanized framework, certain amino acid residues from the mouse framework critical for supporting the CDRs in a binding-capable conformation. Potential criticality is assessed from molecular models. Candidate residues for retention are typically those adjacent in linear sequence to a CDR or physically within 6 Å of any CDR residue.
In other approaches, criticality of particular framework amino acid residues is determined experimentally once a low-avidity humanized construct is obtained, by reversion of single residues to the mouse sequence and assaying antigen binding as described by Riechmann et al, (1988). Another example approach for identifying criticality of amino acids in framework sequences is disclosed by U.S. Pat. No. 5,821,337 to Carter et al, and by U.S. Pat. No. 5,859,205 to Adair et al,. These references disclose specific Kabat residue positions in the framework, which, in a humanized antibody may require substitution with the correspondent mouse amino acid to preserve avidity. One of the disadvantages of the refinements by Queen, and the approaches of Ricechmann, Carter and Adair, is that a very large number of human framework sequences are required for comparison, and/or the guidelines for preserving critical amino acid residues are not completely sufficient to predict functionality. Accordingly, the resulting frameworks constructed, which are part human and part mouse, still frequently exhibit human immunogenicity or lowered antigen binding, thereby requiring numerous iterations in framework construction to obtain a suitable framework for therapeutic uses.
A second type of refinement to Winter is exemplified by Padlan et al (1995) Identification of specificity-determining residues in antibodies, FASEB J. 9:133-139; and Tamura et al (2000) Structural correlates of an anti-carcinoma antibody: identification of specificity-determining residues (SDRs) and development of a minimally immunogenic antibody variant by retention of SDRs only. J. Immunol. 164:1432-1441. These references share the premise that increasing the proportion of characteristically human sequence in a humanized antibody will reduce that antibody's immunogenicity, and accordingly disclose methods for grafting partial CDR sequences. Determination of the three-dimensional structure of antibody-antigen complexes showed that many residue positions assigned to the CDRs defined by Kabat and Wu rarely were directly involved in antigen binding. These references showed that grafting a subset of CDR residues would adequately transfer antigen binding in a humanized antibody. However, humanized framework sequences are still required, and these references do not teach methods for selecting adequate human framework sequences for use with a given set of mouse CDRs.
There is therefore, a need in the art for methods of humanizing antibodies that reliably identify suitable human framework sequences to support non-human CDR regions and to provide humanized antibodies that retain high antigen binding with low immunogenicity in humans, without the need for direct comparison of framework sequences, without need for determining critically important amino acid residues in the framework, and without need for multiple iteration in construction to obtain humanized antibodies with suitable therapeutic properties.